GAS-CHROMATOGRAPHIC DETERMINATION OF ISOCITRIC AND MALIC-ACID IN THE PRESENCE OF A LARGE EXCESS OF CITRIC-ACID

Derivatization yields of different esters (methyl, ethyl, n-propyl, isopropyl, n-butyl and isobutyl) of citric, malic and isocitric acids with various acylating agents, such as acetic anhydride, propionic anhydride, trifluoroacetic anhydride and heptafluorobutyric anhydride, were investigated. The formation of acylated malic and isocitric acid esters is rapid and quantitative whereas the acylation of citric acid esters was slow and partial in most instances. It was found that selective analytical conditions can be achieved with the O-heptafluorobutyryl n-butyl esters. The analytical applicability of the separation and determination of the malic, isocitric and citric acid contents of model solutions and of pressed lemon juice is discussed. Concentrations of 1.2 × 10-3 -9.0 × 10-3g per 100 g of isocitric acid and 4.2 × 10-3 -3.5 × 10-2 g per 100 g of malic acid in the presence of 9.3 × 10-1 g per 100 g of citric acid were measured with relative standard deviations of 7.5 and 4.2%, respectively. © 1990.